Flocculation is a method of dewatering suspended solids by agglomerating the solids. Flocculation materially improves the dewatering rate of many types of suspended solids, including those used in mineral, papermaking, waste water treating and oil field applications.
Synthetic polymer flocculants have been utilized in the industry since the 1950's as flocculating agents in the treatment of suspended solids. However, due to modern concerns with environmental protection, sludge incineration, transportation and disposal costs, it is increasingly desirable to provide polymeric flocculants which can achieve a satisfactory level of dewatering at relatively low dosage levels, as compared with conventional polymeric flocculants.
The present invention provides compositions and methods for dewatering suspended solids, including those frequently encountered in the waste water treating, mining and papermaking industries, using high molecular weight, water-soluble, cationic polymer flocculants, as well as methods for making said compositions.
Linear polymer flocculants have been "structured" in the art through the use of branching or crosslinking agents. Polymer structuring is discussed by J. E. Morgan et al., Adv. Chem. Ser., Vol. 187, pp. 235-52 (1980). U.S. Pat. Nos. 4,720,346 and 4,943,378 describe the use of crosslinked cationic polymer particles having a dry particle size below 10 micrometers (.mu.m).
U.S. Pat. Nos. 5,152,903 and 5,340,865 disclose a method of flocculating using cross-linked cationic polymer microparticles. U.S. Pat. No. 3,235,490 describes a flocculation method which utilizes crosslinked polyacrylamide. U.S. Pat. No. 3,968,037 teaches a method of releasing water from activated sewage sludge using crosslinked cationic emulsion polymers. Methods and compositions useful for thickening aqueous media are given in U.S. Pat. Nos. 4,059,552 and 4,172,066. Pending U.S. application Ser. Nos. 08/028916, 08/028,001, 07/437258, 08/454974 and 08/455419, which are assigned to assignee to this invention and are all hereby incorporated herein by reference, describe methods for flocculating suspended solids using cationic, high molecular weight, water-soluble, branched polymers.
The "sedimentation value" varies as a function of the degree of structuring present in the polymer, and therefore can be utilized as a measure of the degree of structuring in a polymer. The "sedimentation value" is a sensitive indicum of the settling rate in a centrifugal field of a water-soluble or water-swellable polymer in salt solution. A sedimentation value of less than 10% means that there is little or no tendency for the polymer in salt solution to sediment when subjected to a centrifugal field. A sedimentation value is determined by preparing a solution of a particular polymer in NaCl of a defined strength, centrifuging part of the solution for a given length of time and then measuring the polymer concentration, for example from the ultraviolet (UV) absorbance of the uncentrifuged part and of the supernatant of the centrifuged part. A wavelength of 215 nm has been found to be useful for these purposes with acrylamide-containing polymers. The sedimentation value .DELTA.UV represents the absorbance of the centrifuged part compared to the absorbance of the uncentrifuged part and is calculated as .DELTA.UV=[.DELTA.A(uncentrifuged)-.DELTA.A(centrifuged)]/.DELTA.A (uncentrifuged), wherein .DELTA.A=A(polymer solution)-A(salt solution) and A is the measured UV absorbance. The value calculated thereby is multiplied by 100 to give the sedimentation value, which is expressed as a percentage.
It is known in the art to blend polymers of different characteristics in order to provide flocculants of improved characteristics. For instance, a number of workers have proposed blending inverse emulsions of high molecular weight (typically in excess of 1 million) cationic polymers with inverse emulsions of low molecular weight (below 1 million) cationic polymers, to provide flocculants of improved stability (e.g. U.S. Pat. Nos. 5,100,951 and 5,169,540) or improved dewatering properties (e.g. U.S. Pat. Nos. 5,405,554 and 5,643,461).
U.S. Pat. No. 4,943,378 discloses polymeric flocculants which have a specific viscosity above 10 (as measured by a capillary viscometer at 34.degree. C. on a 0.5% solution in deionised water) and which comprise crosslinked water-insoluble, water-swellable polymeric particles that have a dry size of below 10 .mu.m. In one embodiment these particulate flocculants may be made by blending a dissolved polymer with a particulate, generally insoluble polymer, which polymer may be made from the same monomers and differing only in the degree of crosslinking.
U.S. Pat. No. 4,950,725 proposes that the properties of crosslinked, particulate, water-insoluble but water-swellable polymers formed from water-soluble ethylenically unsaturated monomers and monomer blends should be optimised for the intended use of the polymer particles by conducting the polymerisation reaction under uniform reaction conditions, in particular by control of the way in which the crosslinking agent is added and/or the selection of the crosslinking agent(s). By this means, the degree of structure in the final polymer particles can be influenced in a desired direction.
Another approach to providing polymeric flocculants with improved properties is to form multimodal polymeric emulsion systems having two or more different aqueous droplet size distributions or modes, by blending two polymeric emulsions with the required individual characteristics. See, for example, WO95/14728 and the background art discussed therein.
Finally, reference is made to WO97/18167 which discloses the use as flocculants of cationic water-soluble or water-swellable polymers having a bulk viscosity to standard viscosity ratio (BV/SV) of about 300-500 and a sedimentation value about 10% or less. The BV/SV ratio, like the sedimentation value, is a function of the degree of structuring present in the cationic polymers.
In some applications, an acceptable flocculation performance cannot be achieved with linear polymeric flocculants and the use of structured polymers is necessary in order to meet performance specifications. However, structured polymers often need to be employed at higher dosages than linear polymers, and therefore not only involve greater costs but also these higher rates of usage cause increased environmental concerns.